Sound isolating (SI) earphones and headsets are becoming increasingly popular for music listening and voice communication. SI earphones enable the user to hear an incoming audio content signal (be it speech or music audio) clearly in loud ambient noise environments, by attenuating the level of ambient sound in the user ear-canal.
SI earphones benefit from using an ear canal microphone (ECM) configured to detect user voice in the occluded ear canal for voice communication in high noise environments. In such a configuration, the ECM detects sound in the users ear canal between the ear drum and the sound isolating component of the SI earphone, where the sound isolating component is, for example, a foam plug or inflatable balloon. The ambient sound impinging on the ECM is attenuated by the sound isolating component (e.g., by approximately 30 dB averaged across frequencies 50 Hz to 10 kHz). The sound pressure in the ear canal in response to user-generated voice can be approximately 70-80 dB. As such, the effective signal to noise ratio measured at the ECM is increased when using an ear canal microphone and sound isolating component. This is clearly beneficial for two-way voice communication in high noise environments: where the SI earphone wearer with ECM can hear the incoming voice signal reproduced with an ear canal receiver (i.e., loudspeaker), with the incoming voice signal from a remote calling party. Secondly, the remote party can clearly hear the voice of the SI earphone wearer with the ECM even if the near-end caller is in a noisy environment, due to the increase in signal-to-noise ratio as previously described.
The output signal of the ECM with such an SI earphone in response to user voice activity is such that high-frequency fricatives produced by the earphone wearer, e.g., the phoneme /s/, are substantially attenuated due to the SI component of the earphone absorbing the air-borne energy of the fricative sound generated at the user's lips. As such, very little user voice sound energy is detected at the ECM above about 4.5 kHz and when the ECM signal is auditioned it can sound “muffled”.
A number of related art discusses spectral expansion. Application US20070150269 describes spectral expansion of a narrowband speech signal. The application uses a “parameter detector” which for example can differentiate between a vowel and consonant in the narrowband input signal, and generates higher frequencies dependant on this analysis.
Application US20040138876 describes a system similar to US20070150269 in that a narrowband signal (300 Hz to 3.4 kHz) is analysis to determine in sibilants or non-sibilants, and high frequency sound is generated in the case of the former occurrence to generate a new signal with energy up to 7.7 kHz.
U.S. Pat. No. 8,200,499 describes a system to extend the high-frequency spectrum of a narrow-band signal. The system extends the harmonics of vowels by introducing a non-linearity. Consonants are spectrally expanded using a random noise generator.
U.S. Pat. No. 6,895,375 describes a system for extending the bandwidth of a narrowband signal such as a speech signal. The method comprises computing the narrowband linear predictive coefficients (LPCs) from a received narrowband speech signal and then processing these LPC coefficients into wideband LPCs, and then generating the wideband signal from these wideband LPCs